Exclude users from annotation sessions

[mzur]

This is related to #12.

Evaluate the possibilities to exclude users from annotation sessions. The user stroy is the following:

Users A, B and C want to conduct an annotation session and measure inter observer agreement in transect T. Meanwhile user D is already working on a specific task, say annotating laserpoints for all images of T. Once the annotation session started, user D no longer sees their annotations created before the annotation session started because all users are affected by the annotation session. Now user D may not know which images and laserpoints they already annotated and may cause them to produce duplicate annotations.

@dlangenk Is the user story accurate?

One solution may be to restrict the annotation session to a set of users. All other users are not affected by the annotation session. Another option is to exclude users from the annotation session. All but the excluded users are affected by the session.

Excluded users are not affected by the "hide own annotations" and "hide other users' annotations" options of an active annotation session. Also, if a report is generated for an annotation session, annotations of excluded users are not added to the report (and vice versa: if the annotation session is excluded from the report, only annotations of excluded users are added to the report, this is related to report filters). Analogously the annotation filter of the annotation tool also respects the users of the annotation session.

[mzur]

I've now chosen the opt-in variant because I think it is easier to understand for the users and least likely to cause conflicts.

Annotation sessions are only active for users belonging to the session. The annotation session filter of the annotation tool shows only annotations belonging to the time span and users of the annotation session. Analogously reports restricted to an annotation session now only include annotations created by users belonging to the session.