Add ability to filter projects when using BitBucket Cloud importer

Problem to solve

Currently, the Bitbucket Cloud 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.

This issue is similar to one for Bitbucket Server importing. https://gitlab.com/gitlab-org/gitlab-ce/issues/51260

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.