lakefs new repositories with import branch as parent branch by default

[nopcoder]

Closes #787

1. Remove import UI and API
2. Create repository will create import branch
3. lakefs import will verify that the import branch was created and not create a branch for you - it means that only new repositories will be valid for import
4. The new repositories will have the default (master) branch be a child branch of the import branch, so merge from the parent will be performed on merge from import.

[codecov-io]

Codecov Report

Merging #810 into master will increase coverage by 0.27%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
+ Coverage   41.88%   42.15%   +0.27%     
==========================================
  Files         133      134       +1     
  Lines       10422    10402      -20     
==========================================
+ Hits         4365     4385      +20     
+ Misses       5477     5432      -45     
- Partials      580      585       +5     

Impacted Files	Coverage Δ
catalog/cataloger.go	68.14% <ø> (ø)
catalog/cataloger_set_default_branch.go	55.55% <55.55%> (ø)
catalog/cataloger_create_repository.go	62.16% <86.66%> (+5.01%)	⬆️
api/api_controller.go	37.47% <100.00%> (+1.34%)	⬆️
catalog/cataloger_commit.go	78.78% <0.00%> (-3.04%)	⬇️
onboard/import.go	62.74% <0.00%> (-2.57%)	⬇️
onboard/catalog_actions.go	67.85% <0.00%> (-2.28%)	⬇️
block/s3/inventory_iterator.go	85.50% <0.00%> (+0.89%)	⬆️

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