Reset cursor for remote KB

Purpose:
Sometimes a user wants to trigger a reharvest of a remote KB from scratch - and this requires the remote_kb cursor to be reset / made blank

User story statement(s):

As a ERM administrator
I want to be able to clear the cursor on a remote_kb
so that the next harvest will harvest all records from the remote_kb

Scenarios:

1. Scenario:
   • Given Settings -> Local KB Admin -> External Data sources
   • When I view an External Data Source record
   • Then I want to see an "Actions" button dropdown menu per external data source
2. Scenario:
   • Given the Actions button on an External Data source record
   • When I click the Actions button
   • Then I see three options: Edit, Reset cursor, Delete
3. Scenario:
   • Given the Actions button on an External Data source record
   • When I select Edit
   • Then the record switches to Edit mode as currently with Cancel/Save options as currently
4. Scenario:
   • Given the Actions button on an External Data source record
   • When I select Delete
   • Then I get a Delete confirmation prompt as currently
5. Scenario:
   • Given the Actions button on an External Data source record
   • When I select Reset cursor
   • Then the value of the cursor is set to null

Dev task breakdown

• These are essentially individual PUTs on the same endpoint as the EDIT, but for two special fields
• We don't want the user entering whatever they want, so these buttons will instead trigger a PUT for those fields individually with our pre-set "reset" values.
• Add an actions dropdown to the KB admin edit in settings
• Add a `handleResetCursor` method (and likewise for resetting sync status) which will handle the actual PUT, can be a modified version of the current submit handler
• Wire up the action button to this handler