Improved gallery cover lookup (#3391)

* Gallery cover lookup by regex in config.yml
* Added regex validation and an in-app manual entry
* Improved settings description + some translations
* Add changelog entry

ui/v2.5/src/docs/en/Changelog/v0200.md

@@ -1,4 +1,5 @@
 ### ✨ New Features
+* Support customising the filename regex used for determining the gallery cover image. ([#3391](https://github.com/stashapp/stash/pull/3391))
 * Added tenth-place rating precision option. ([#3343](https://github.com/stashapp/stash/pull/3343))
 * Added toggleable favorite button to Performer cards. ([#3369](https://github.com/stashapp/stash/pull/3369))
 

ui/v2.5/src/docs/en/Manual/Configuration.md

@@ -129,6 +129,7 @@ These options are typically not exposed in the UI and must be changed manually i
 | `custom_ui_location` | The file system folder where the UI files will be served from, instead of using the embedded UI. Empty to disable. Stash must be restarted to take effect. |
 | `max_upload_size` | Maximum file upload size for import files. Defaults to 1GB. |
 | `theme_color` | Sets the `theme-color` property in the UI. |
+| `gallery_cover_regex` | The regex responsible for selecting images as gallery covers |
 | `proxy` | The url of a HTTP(S) proxy to be used when stash makes calls to online services Example: https://user:password@my.proxy:8080 |
 | `no_proxy` | A list of domains for which the proxy must not be used. Default is all local LAN: localhost,127.0.0.1,192.168.0.0/16,10.0.0.0/8,172.16.0.0/12 |