Add user hint to save new keyword values

[ottonomy]

Closes #112

I rejected the suggested approach to just change the gray text, because would go against UX patterns. The text is supposed to suggest a possible value that might be entered, rather than provide instruction. But the underlying issue was correct: the user didn't know that they could create a new keyword by hitting Enter.

Now, when relevant, a new button appears in the search result list. When a user selects it (or hits their "Enter" key while the input control is selected, the current search value is set to be saved as a new keyword value. This button appears when:

• A search term has been entered, but no corresponding result is found
• A search term has been entered, and some results have been returned, but none of the results exactly matches the search term.

It does not appear when:

• a search term has been entered that is an exact match of a found result.

[ottonomy]

@manuel-delvillar thanks! Fixed that in cef764e.