Allow deletion of multiple missions at once

[McDiod]

Our use case is dynamic missions that can be played on multiple maps. Whenever a new version would be released, you'd have to manually delete the previous version's files for every single map.

[nomisum]

maybe even cluster missions with same prefix (prefix.islandname) into collapsible containers, because it makes no sense to delete one version but not the others 👍

(besides would make the list much shorter und more easily digestable)

[Dahlgren]

Would probably be a good idea to add optional grouping per island as well.

Great idea @nomisum!

[Fusselwurm]

maybe even cluster missions with same prefix (prefix.islandname) into collapsible containers

a bit more versatile:

• have a checkbox next to each mission
• have a search bar to filter the list (dynamically while typing etc)
• a "select all" checkbox that checks all currently shown missions
• and a "delete selected" button

[nomisum]

i would additionally love a

• move all except selected ones to temporary nirvana space button

(use case: #missions takes ages due to amount of missions available, temporarily moving files would make this easy)

[Dahlgren]

With the upgrade to newer Marionette there is built in support for filtering so I think I'll postpone this until then

[Dahlgren]

Search bar with filter and showing mission and worlds name as separate columns is now implemented in feature/missions for both missions page and server settings. Checkboxes, mass delete and maybe grouping left I guess.