Description of problem:
Most of the multi-select lists have been changed to Selectize, the Keywords field has not.

For Keywords, you get a drop-down to select the first value, then the list closes and you have to type something to select additional values -- clicking on the field doesn't open the drop-down again if at least one value is alredy selected.

For consistency, Keywords should use a Selectize list like the other drop-downs.

Version-Release number of selected component (if applicable):
5.0.3

How reproducible:
Always

Steps to Reproduce:
1. Go to New->Community->Bugzilla
2. Select a Keyword using only the mouse
3. Try to select a second Keyword using only the mouse

Actual results:
In step 3, it seems impossible to select a second keyword using the mouse alone.

Expected results:
Keywords behaves consistently with other multi-select lists in the Bugzilla UI -- any number of Keywords can be selected using only the mouse.

Additional info:
none