Add Matrix webhook

[S7evinK]

This PR adds the possibility to add Matrix webhooks as requested in #6590.
Since the the URL displayed is a little bit long (spanning multiple lines), we might need to change

gitea/templates/repo/settings/webhook/list.tmpl

Line 50 in b1c331c

<a class="dont-break-out" href="{{$.BaseLink}}/{{.ID}}">{{.URL}}</a>

to contain a trimmed version of the URL to make it look better, or is there a better way to only display up to x chars of the URL?

[lafriks]

Did not test but looks good ;)

[S7evinK]

I did some tests myself:

And like mentioned, the Webhooks overview doesn't look that nice with very long URLs:

[codecov-io]

Codecov Report

Merging #10831 into master will decrease coverage by 0.09%.
The diff coverage is 20.55%.

@@            Coverage Diff             @@
##           master   #10831      +/-   ##
==========================================
- Coverage   43.49%   43.40%   -0.10%     
==========================================
  Files         592      593       +1     
  Lines       82979    83265     +286     
==========================================
+ Hits        36094    36141      +47     
- Misses      42399    42632     +233     
- Partials     4486     4492       +6     

Impacted Files	Coverage Δ
models/webhook.go	65.97% <0.00%> (-0.35%)	⬇️
modules/auth/repo_form.go	43.69% <0.00%> (-0.75%)	⬇️
modules/webhook/deliver.go	45.16% <0.00%> (-1.25%)	⬇️
modules/webhook/webhook.go	42.36% <0.00%> (-1.53%)	⬇️
routers/repo/webhook.go	1.11% <0.00%> (-0.12%)	⬇️
modules/webhook/matrix.go	28.88% <28.88%> (ø)
modules/setting/webhook.go	56.25% <100.00%> (ø)
routers/routes/routes.go	86.47% <100.00%> (+0.09%)	⬆️
modules/indexer/stats/db.go	40.62% <0.00%> (-18.75%)	⬇️
modules/indexer/stats/queue.go	62.50% <0.00%> (-18.75%)	⬇️
... and 11 more

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[tulir]

Matrix events accept arbitrary JSON, so it might be nice to send some raw webhook data too. That way it could be used as a full alternative for traditional webhooks instead of just a chat thing. The only limit is that events can't be more than ~64 KiB, not sure how likely webhook data is to hit that.

Something like this:

{
  "msgtype": "m.text",
  "body": "...",
  "io.gitea.webhook": {
    "commits": [...]
  }
}

[lafriks]

As per comments above

[S7evinK]

Using the Authorization header feels like a dirty workaround. Couldn't figure out a better solution.

[OahzEgroeg]

Can't add matrix as default webhooks or system webooks from site administration, when select matrix, it doesn't return any form.

[techknowlogick]

@EgroegOahz please create a ticket for this.