Add support for federated avatars (#3320)

* Add support for federated avatars

Fixes #3105

Removes avatar fetching duplication code
Adds an "Enable Federated Avatar" checkbox in user settings
(defaults to unchecked)

Moves avatar settings all in the same form, making
local and remote avatars mutually exclusive

Renames UploadAvatarForm to AvatarForm
as it's not anymore only for uploading

* Run gofmt on all modified files

* Move Avatar form in its own page

* Add go-libravatar dependency to vendor/ dir

Hopefully helps with accepting the contribution.
See also #3214

* Revert "Add go-libravatar dependency to vendor/ dir"

This reverts commit a8cb93ae640bbb90f7d25012fc257bda9fae9b82.

* Make federated avatar setting a global configuration

Removes the per-user setting

* Move avatar handling back to base tool, disable federated avatar in offline mode

* Format, handle error

* Properly set fallback host

* Use unsupported github.com mirror for importing go-libravatar

* Remove comment showing life exists outside of github.com

... pity, but contribution would not be accepted otherwise

* Use Combo for Get and Post methods over /avatar

* FEDERATED_AVATAR -> ENABLE_FEDERATED_AVATAR

* Fix persistance of federated avatar lookup checkbox at install time

* Federated Avatars -> Enable Federated Avatars

* Use len(string) == 0 instead of string == ""

* Move import line where it belong

See
https://github.com/Unknwon/go-code-convention/blob/master/en-US/import_packages.md

Pity the import url is still the unofficial one, but oh well...

* Save a line (and waste much more expensive time)

* Remove redundant parens

* Remove an empty line

* Remove empty lines

* Reorder lines to make diff smaller

* Remove another newline

Unknwon review got me start a fight against newlines

* Move DISABLE_GRAVATAR and ENABLE_FEDERATED_AVATAR after OFFLINE_MODE

On re-reading the diff I figured what Unknwon meant here:
https://github.com/gogits/gogs/pull/3320/files#r73741106

* Remove newlines that weren't there before my intervention

conf/app.ini

@@ -230,6 +230,9 @@ AVATAR_UPLOAD_PATH = data/avatars
 ; or a custom avatar source, like: http://cn.gravatar.com/avatar/
 GRAVATAR_SOURCE = gravatar
 DISABLE_GRAVATAR = false
+; Federated avatar lookup uses DNS to discover avatar associated
+; with emails, see http://www.libravatar.org
+ENABLE_FEDERATED_AVATAR = false
 
 [attachment]
 ; Whether attachments are enabled. Defaults to `true`

conf/locale/locale_en-US.ini

@@ -96,6 +96,7 @@ offline_mode = Enable Offline Mode
 offline_mode_popup = Disable CDN even in production mode, all resource files will be served locally.
 disable_gravatar = Disable Gravatar Service
 disable_gravatar_popup = Disable Gravatar and custom sources, all avatars are uploaded by users or default.
+federated_avatar_lookup = Enable Federated Avatars Lookup
 disable_registration = Disable Self-registration
 disable_registration_popup = Disable user self-registration, only admin can create accounts.
 enable_captcha = Enable Captcha
@@ -239,6 +240,7 @@ form.name_pattern_not_allowed = Username pattern '%s' is not allowed.
 [settings]
 profile = Profile
 password = Password
+avatar = Avatar
 ssh_keys = SSH Keys
 social = Social Accounts
 applications = Applications
@@ -259,7 +261,9 @@ change_username_prompt = This change will affect the way how links relate to you
 continue = Continue
 cancel = Cancel
 
-enable_custom_avatar = Enable Custom Avatar
+lookup_avatar_by_mail = Lookup Avatar by mail
+federated_avatar_lookup = Federated Avatar Lookup
+enable_custom_avatar = Use Custom Avatar
 choose_new_avatar = Choose new avatar
 update_avatar = Update Avatar Setting
 delete_current_avatar = Delete Current Avatar
@@ -1044,6 +1048,7 @@ config.cookie_life_time = Cookie Life Time
 config.picture_config = Picture Configuration
 config.picture_service = Picture Service
 config.disable_gravatar = Disable Gravatar
+config.enable_federated_avatar = Enable Federated Avatars
 config.log_config = Log Configuration
 config.log_mode = Log Mode