Repository avatar fallback configuration (#7087)

* Only show repository avatar in list when one was selected

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Adds fallback configuration option for repository avatar

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Implements repository avatar fallback

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Adds admin task for deleting generated repository avatars

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Solve linting issues

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Save avatar before updating database

* Linting

* Update models/repo.go

Co-Authored-By: zeripath <art27@cantab.net>

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

@@ -292,6 +292,11 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
    [http://www.libravatar.org](http://www.libravatar.org)).
 - `AVATAR_UPLOAD_PATH`: **data/avatars**: Path to store user avatar image files.
 - `REPOSITORY_AVATAR_UPLOAD_PATH`: **data/repo-avatars**: Path to store repository avatar image files.
+- `REPOSITORY_AVATAR_FALLBACK`: **none**: How Gitea deals with missing repository avatars
+  - none = no avatar will be displayed
+  - random = random avatar will be generated
+  - image = default image will be used (which is set in `REPOSITORY_AVATAR_DEFAULT_IMAGE`)
+- `REPOSITORY_AVATAR_FALLBACK_IMAGE`: **/img/repo_default.png**: Image used as default repository avatar (if `REPOSITORY_AVATAR_FALLBACK` is set to image and none was uploaded)
 - `AVATAR_MAX_WIDTH`: **4096**: Maximum avatar image width in pixels.
 - `AVATAR_MAX_HEIGHT`: **3072**: Maximum avatar image height in pixels.
 - `AVATAR_MAX_FILE_SIZE`: **1048576** (1Mb): Maximum avatar image file size in bytes.