Add pagination and search in landingzones file list modal

[mikkonie]

@Nicolai-vKuegelgen recently did a bit of stress testing on the landing zones in production. As a result, I've encountered many interesting things which probably should've been thought of while developing. See also #1872.

Suffice to say, it turns out that attempting to retrieve and display a flat list of 170 000 files does not result in a decent outcome with the current implementation :)

The landingzones UI is due to a full overhaul and with that, a more decent file browsing modal view should be added.

In the meantime, we could at least do the following:

• Add pagination
• Have filtering search for all files (requires new ajax view)

TODO: Write a full spec

Related to #1730, which should be done first.

[mikkonie]

Postponed this to v1.1, to be done together with #1996 and #1997.