fix(webdav): Set Retry-After header for 429 responses

We're now setting a 'Retry-After' in the webdav response when the
thunbnailer is rate limiting the requests. For now the timeout is just
a random number between 1 and 15 seconds.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f