Add Webhook authorization header (#20926)

_This is a different approach to #20267, I took the liberty of adapting
some parts, see below_

## Context

In some cases, a weebhook endpoint requires some kind of authentication.
The usual way is by sending a static `Authorization` header, with a
given token. For instance:

- Matrix expects a `Bearer <token>` (already implemented, by storing the
header cleartext in the metadata - which is buggy on retry #19872)
- TeamCity #18667
- Gitea instances #20267
- SourceHut https://man.sr.ht/graphql.md#authentication-strategies (this
is my actual personal need :)

## Proposed solution

Add a dedicated encrypt column to the webhook table (instead of storing
it as meta as proposed in #20267), so that it gets available for all
present and future hook types (especially the custom ones #19307).

This would also solve the buggy matrix retry #19872.

As a first step, I would recommend focusing on the backend logic and
improve the frontend at a later stage. For now the UI is a simple
`Authorization` field (which could be later customized with `Bearer` and
`Basic` switches):


![2022-08-23-142911](https://user-images.githubusercontent.com/3864879/186162483-5b721504-eef5-4932-812e-eb96a68494cc.png)

The header name is hard-coded, since I couldn't fine any usecase
justifying otherwise.

## Questions

- What do you think of this approach? @justusbunsi @Gusted @silverwind 
- ~~How are the migrations generated? Do I have to manually create a new
file, or is there a command for that?~~
- ~~I started adding it to the API: should I complete it or should I
drop it? (I don't know how much the API is actually used)~~

## Done as well:

- add a migration for the existing matrix webhooks and remove the
`Authorization` logic there


_Closes #19872_

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: delvh <dev.lh@web.de>

options/locale/locale_en-US.ini

@@ -2012,6 +2012,8 @@ settings.event_package = Package
 settings.event_package_desc = Package created or deleted in a repository.
 settings.branch_filter = Branch filter
 settings.branch_filter_desc = Branch whitelist for push, branch creation and branch deletion events, specified as glob pattern. If empty or <code>*</code>, events for all branches are reported. See <a href="https://pkg.go.dev/github.com/gobwas/glob#Compile">github.com/gobwas/glob</a> documentation for syntax. Examples: <code>master</code>, <code>{master,release*}</code>.
+settings.authorization_header = Authorization Header
+settings.authorization_header_desc = Will be included as authorization header for requests when present. Examples: %s.
 settings.active = Active
 settings.active_helper = Information about triggered events will be sent to this webhook URL.
 settings.add_hook_success = The webhook has been added.
@@ -2126,7 +2128,6 @@ settings.bot_token = Bot Token
 settings.chat_id = Chat ID
 settings.matrix.homeserver_url = Homeserver URL
 settings.matrix.room_id = Room ID
-settings.matrix.access_token = Access Token
 settings.matrix.message_type = Message Type
 settings.archive.button = Archive Repo
 settings.archive.header = Archive This Repo