[C2] Document variation per URL

[ywarnier]

This is a very multi-url-specific feature.
When managing many URLs, you might want to re-use courses from the main URL with all URLs (through sessions).
However, wouldn't it be great to be able, for some specific documents (HTML, PDF, video, etc) to be able to manage variations per URL, such that loading the same document (e.g. a PDF) from a different URL would return a slightly different document (e.g. with a different logo).

This would require:

• the ResourceFile entity to include a Url attribute (null by default)
• the addition of a new configuration setting "access_url_specific_files" (off by default)
• a new or modified listener (or viewer script) that checks first if access_url_specific_files is on, and if so, looks for a URL-specific resource_file. If this URL-specific file does not exist, use the one with url=null
• a new icon (MDI's file-replace, added as ActionIcon::ADD_FILE_VARIATION) in the documents list (only if access_url_specific_files is on) to add a new version of the file
• this new page should show an upload option + a list of all current versions (with an indication of the format and the URL)

[christianbeeznest]

Added support for URL-specific document variations:

Added AccessUrl to ResourceFile entity.
Introduced access_url_specific_files setting.
Updated controllers to handle URL-specific files.
Enhanced Vue component for uploading and displaying variations.
Improved routing and UI for managing variations.