[Migration Experience][1.5.1] Update the refresh buttons.

[krritik]

Bug - https://dev.azure.com/msdata/Database%20Systems/_workitems/edit/2806472

Removed the dropdown for RefreshAssessment and RefreshSKU. Instead both are now as separate buttons in the toolbar -

1. On PageEnter (disabled)
   
2. On Import Perf Data (disabled)
   
3. On start data collection (enabled)
   

[coveralls]

Pull Request Test Coverage Report for Build 6863974226

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.005%) to 41.57%

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Files with Coverage Reduction	New Missed Lines	%
extensions/notebook/src/book/bookTreeView.ts	1	36.91%
extensions/sql-database-projects/src/common/httpClient.ts	1	66.18%
extensions/sql-database-projects/src/dialogs/publishDatabaseDialog.ts	2	52.03%

Totals
Change from base Build 6860572786:	-0.005%
Covered Lines:	30785
Relevant Lines:	69355

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💛 - Coveralls