Use ETag / If-Non-Match when serving (more or less) static content

[dariok]

This will be mostly true for JS and CSS as well as images — at least in edoc/resources and data/resources; ideally, also in a project’s resources.

• create a new file in edoc/index to store hashes for these files
• use the update trigger (as that is active anyway) to enter the current hash for a file when updating it
• when serving a file (at least from $shared and $global), add an ETag header
• when receiving a request with an If-Non-Match header, evaluate it against the hash
• try to extend this to project resources (see $r-{$projectID} → {$wdb:data}/{$projectCollection}/resources (?) #616)