Add ability to filter projects when using BitBucket Server importer

Problem to solve

Currently, the Bitbucket Server Importer has an “Import all projects”, but for customers with a lot of projects, it might cause issues to import all the projects due to performance, or automatic namespace creation, among other things.

Proposal

• Filter the projects import page by a string.
  • Only list the projects that match a portion of the string. The user can then click "Import" next to the desired projects.

We should apply this pattern to all importers, but not for an MVC if it takes considerably more LOE.

WIP from @georgekoltsov

Links / references

https://gitlab.zendesk.com/agent/tickets/103368

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