Allow groups to be re-imported

What does this MR do?

Original issue: #299498 (closed)

Allows groups to be imported multiple times

Previously after importing group once UI was disabled. Lift that requirement

Screenshots or Screencasts (strongly suggested)

Import_groups___GitLab

• Enable feature bulk_import via Feature.enable(:bulk_import)
• Open "New group" (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)
• Modify any dropdown to target existing project (note: right now this is case sensitive, it is known issue and out of scope of this MR)

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • Changelog is not needed, feature is behind feature flag
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.