Search range slider fix and improvement

[litvinovg]

• Other Relevant Links (Mailing list discussion, related pull requests, etc.)

What does this pull request do?

Fixes missing label for selected date range.
Adds input fields for date range slider.

How should this be tested?

• Put configuration for date range filter into rdf/display/firsttime and remove .txt suffix from file name
• Load sample data
• Select a range, try manually modifying start and end values.
• Verify that there is a button to remove selected filter on search page.

Interested parties

@VIVO-project/vivo-committers

Reviewers' expertise

Candidates for reviewing this PR should have some of the following expertises:

1. HTML, CSS, JavaScript
2. FreeMarker

Reviewers' report template

General comment

A reviewer should provide here comments and suggestions for requested changes if any.

Testing

A reviewer should briefly describe here how it was tested

Code reviewing

A reviewer should briefly describe here which part was code reviewed

[chenejac]

@litvinovg it looks ok for me, and it works for me in accordance with the expectations. I have followed testing instructions.