Create DB session provider(based on xorm) (#13031)

* Create Xorm session provider

This PR creates a Xorm session provider which creates
the appropriate Session table for macaron/session.

Fix #7137

Signed-off-by: Andrew Thornton <art27@cantab.net>

* extraneous l

Signed-off-by: Andrew Thornton <art27@cantab.net>

* fix lint

Signed-off-by: Andrew Thornton <art27@cantab.net>

* use key instead of ID to be compatible with go-macaron/session

Signed-off-by: Andrew Thornton <art27@cantab.net>

* And change the migration too.

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Update spacing of imports

Co-authored-by: 6543 <6543@obermui.de>

* Update modules/session/xorm.go

Co-authored-by: techknowlogick <matti@mdranta.net>

* add xorm provider to the virtual provider

Signed-off-by: Andrew Thornton <art27@cantab.net>

* prep for master merge

* prep for merge master

* As per @lunny

* move migration out of the way

* Move to call this db session as per @lunny

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -551,7 +551,7 @@ Define allowed algorithms and their minimum key length (use -1 to disable a type
 
 ## Session (`session`)
 
-- `PROVIDER`: **memory**: Session engine provider \[memory, file, redis, mysql, couchbase, memcache, postgres\].
+- `PROVIDER`: **memory**: Session engine provider \[memory, file, redis, db, mysql, couchbase, memcache, postgres\].
 - `PROVIDER_CONFIG`: **data/sessions**: For file, the root path; for others, the connection string.
 - `COOKIE_SECURE`: **false**: Enable this to force using HTTPS for all session access.
 - `COOKIE_NAME`: **i\_like\_gitea**: The name of the cookie used for the session ID.