Update ImportProjectsTable component

What does this MR do?

Related issue: #26326 (closed)

This MR updates our ImportProjectsTable with functionality missing for displaying "incompatible repos" (which are the case for Bitbucket and Bitbucket Server importers. Key functionalities:

• ability to display incompatible repos rows
• slot to display warning message (message will be provided by each importer independently)

Also it does some background tasks:

• refactor Vuex according to latest RFCs
• update tests to be actually testing original component
• Switch buttons to gitlab-ui

Screenshots

item	before	after
Header
Incompatible project rows

(please take a note - alert design & colors are outside scope of this issue, included for demo purposes only)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done