Add support for custom avatars (via file path) and for not touching the already-set avatar

This is based on the work done in https://github.com/etkecc/baibot/pull/60 by https://github.com/Fmstrat (Ben Curtis),
with various changes on top to make the code more idiomatic and flexible.

This commit squashes the following patches (newest first):

- Improve handling of `user.avatar` configuration (null & empty string being the same now) and add support for a special `keep` value
- Minor import reordering
- Simplify avatar configuration (`user.avatar.source` -> `user.avatar`)
- Combine `logo_bytes` and `mime_type` determination logic and do not fall back to default avatar if reading the custom avatar file fails
- Switch from deprecated `mime_guess::guess_mime_type(avatar_path)` to `mime_guess::from_path(avatar_path).first_or_octet_stream()`
- Relax `mime_guess` constraint and order alphabetically
- Use `mime` from `mxlink`
- Add mime-type support and switch to user.avatar.source
- (Original work by Fmstrat) Add support for custom avatars

Co-authored-by: Fmstrat <nospam@nowsci.com>

etc/app/config.yml.dist

@@ -11,6 +11,12 @@ user:
   # Leave empty to use the default (baibot).
   name: baibot
 
+  # An optional path to an image file to be used as a custom avatar image.
+  # - null or empty string: use the default avatar
+  # - "keep": don't touch the avatar, keep whatever is already set
+  # - any other value: path to a custom avatar image file
+  avatar: null
+
   encryption:
     # An optional passphrase to use for backing up and recovering the bot's encryption keys.
     # You can use any string here.