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>

src/entity/cfg/env.rs

@@ -6,6 +6,7 @@ pub const BAIBOT_HOMESERVER_URL: &str = "BAIBOT_HOMESERVER_URL";
 pub const BAIBOT_USER_MXID_LOCALPART: &str = "BAIBOT_USER_MXID_LOCALPART";
 pub const BAIBOT_USER_PASSWORD: &str = "BAIBOT_USER_PASSWORD";
 pub const BAIBOT_USER_NAME: &str = "BAIBOT_USER_NAME";
+pub const BAIBOT_USER_AVATAR: &str = "BAIBOT_USER_AVATAR";
 pub const BAIBOT_USER_ENCRYPTION_RECOVERY_PASSPHRASE: &str =
     "BAIBOT_USER_ENCRYPTION_RECOVERY_PASSPHRASE";
 pub const BAIBOT_USER_ENCRYPTION_RECOVERY_RESET_ALLOWED: &str =