Add ability to add bulk users to a group

[Daniel-WWU-IT]

This PR adds the ability to perform a "bulk import" of users to an existing group. A simple multiline textfield can be used to enter several user names and add them to the selected group with a single click. Handy if you want need to add 100+ users to a single group: Copy & paste their user names into the new edit field, click "Add bulk users" and you are done. Successfully tested on Sciebo.

[CLAassistant]

All committers have signed the CLA.

[micbar]

@Daniel-WWU-IT Why close?

[Daniel-WWU-IT]

I realized that I had "circumvented" using your handlebars templates and edited the resulting JS file instead (was easier for me, as this extension was done in 2 or 3 hours about 3 months ago). I suppose that a proper PR requires me to avoid this little detour, so I closed it for now; maybe I'll find the time to take a look at this handlebar stuff and reopen this PR.

[sonarcloud[bot]]

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[DeepDiver1975]

We have meanwhile a csv import and export capability which suits this use case better form our understanding. THX a lot