Tasks: assign tags to users

[kdarras]

A new table should be created to allow users to assign other users to review some tags.

The table "assignments" should probably contain:

• assigned by user id
• assigned to user id
• datetime assigned
• tag id
• comment
• status (assigned/reviewed)

Inside the tag window, a new button ("assign reviewers") should be added. This should bring up a prompt where multiple users who have review privileges for the corresponding recording can be selected (similar to acoustic indices dropdown menu). A comment field with optional text should also be given.

For assigning multiple tags to multiple users, it should be possible to select multiple tags and assign users with a button above the tags table (similarly as for sites and user privileges).

The assignments should show up under a new tab in the dashboard and users should see only their assignments and the assignments that they gave. The users should be able to open the corresponding tag (after checking the line and clicking the View button). Editing the status should only be possible when you were assigned the corresponding task.

The usual functionalities should be offered for this table.

[kdarras]

Added in response to #560
To help users process their assignment in one batch, a playback button should be offered, which would open a new spectrogram window with the tag window open for the first tag. Instead of going to the next tag in the recording, the "next" button should go to the next tag in the assignments table.