RCLFilters: UI tweaks

RCLFilters: UI tweaks

Follow-up to I96af7ba583d03e6ff9833ac3b5f4b80cfd0ee626

* Reword 'Recentchangeslinked-summary'
* Bold ToOrFrom selector label
* Bold 'FROM' in 'rcfilters-filter-showlinkedfrom-option-label'
* Bold 'TO' in 'rcfilters-filter-showlinkedto-option-label'
* Reword target page selector placeholder
* Prevent title suggestions from showing duplicate entries
* Show a specific message when there is no results
  because no target page is specified.

Bug: T172161
Change-Id: I1b1ee793b3eff4183039acc27fe6cca042ee57d0