Accessibility fixes for confluence-search bundled plugin

Problem Definition

Currently there are a few accessibility issues with the templates for this plugin, it is missing some accessibility code:

• search.html.vm
  Add label to search input and aria-label to filter.
• cql-search-result.soy
  Change h3 tag to h2
  Add noresults.login text to inform the user to login to receive full results
• confluence-search.properties
  Added noresults.login text