Refactor authors dropdown (send get request from frontend to avoid long wait time) (#23890)

Right now the authors search dropdown might take a long time to load if
amount of authors is huge.
Example: (In the video below, there are about 10000 authors, and it
takes about 10 seconds to open the author dropdown)

https://user-images.githubusercontent.com/17645053/229422229-98aa9656-3439-4f8c-9f4e-83bd8e2a2557.mov

Possible improvements can be made, which will take 2 steps (Thanks to
@wolfogre for advice):

Step 1：
Backend: Add a new api, which returns a limit of 30 posters with matched
prefix.
Frontend: Change the search behavior from frontend search(fomantic
search) to backend search(when input is changed, send a request to get
authors matching the current search prefix)

Step 2：
Backend: Optimize the api in step 1 using indexer to support fuzzy
search.

This PR is implements the first step. The main changes:
1. Added api: `GET /{type:issues|pulls}/posters` , which return a limit
of 30 users with matched prefix (prefix sent as query). If
`DEFAULT_SHOW_FULL_NAME` in `custom/conf/app.ini` is set to true, will
also include fullnames fuzzy search.
2. Added a tooltip saying "Shows a maximum of 30 users" to the author
search dropdown
3. Change the search behavior from frontend search to backend search

After:

https://user-images.githubusercontent.com/17645053/229430960-f88fafd8-fd5d-4f84-9df2-2677539d5d08.mov

Fixes: https://github.com/go-gitea/gitea/issues/22586

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

options/locale/locale_en-US.ini

@@ -933,6 +933,7 @@ delete_preexisting = Delete pre-existing files
 delete_preexisting_content = Delete files in %s
 delete_preexisting_success = Deleted unadopted files in %s
 blame_prior = View blame prior to this change
+author_search_tooltip = Shows a maximum of 30 users
 
 transfer.accept = Accept Transfer
 transfer.accept_desc =  Transfer to "%s"