Improve YANG Search UX

[richardzilincikPantheon]

Based on an email from Benoit, there are several issues with the search UX.

• If no search term is entered, a search request can still be made, returning a 400 error. An error is returned even if advanced search options have been specified.

• Advanced search options apply on top of the other specified search options, but this isn't obvious from the UI, and it could be assumed they operate independently.

• The names of advanced options use internal terminology and have no descriptions attached, making it difficult for the user to correctly specify what they're looking for.

• A "Maximum number of results reached" message may be shown even if some or all results were filtered out based on advanced search options, and were not displayed to the user.

[richardzilincikPantheon]

@bclaise what you were trying to search can currently be done by:

1. clearing the advanced search options
2. selecting only "Typedef" in the Schema Types tab
3. type the query into the main search field

We'll work towards resolving the problems mentioned in this issue and making the UI easier to use.