Updates RBSearchView and adds the templates, CSS, and datagrid

infrastructure for the new faceted search results page.
RBSearchView.dispatch now checks whether the active backend exposes

get_es_client. If it does, a FacetedSearchEngine is constructed and

_use_faceted is set to True. Any exception during construction falls

back silently to the existing Haystack path. Numeric queries still

redirect directly to the matching review request when accessible.
get_template_names returns search/results.html for the faceted path

and search/results_legacy.html for the Haystack path.
get_context_data on the faceted path calls

FacetedSearchEngine.search, builds the group sidebar items with total

counts, resolves human-readable labels for searchable filter buckets

(reviewer, group, repository), and assembles the filter panel context

including current values, bucket lists, and option counts.
Search results are displayed using Review Board's existing datagrid

infrastructure. Three new datagrid classes are added:

SearchReviewRequestDataGrid, SearchUsersDataGrid, and

SearchGroupDataGrid. Each takes an ES-ordered PK list and an ES total

count, builds a Case/When-ordered ORM queryset from those PKs, and

overrides pagination to use the ES total rather than a COUNT(*). Letter

navigation for users and groups is handled server-side by injecting a

prefix or regexp filter clause directly into the ES query, avoiding

any full result fetch regardless of dataset size.
results.html is the new template for the faceted path, providing the

filter sidebar and results list. results_legacy.html is the existing

template renamed, unchanged in behaviour. search_listview.html and

search_paginator.html are new search-specific datagrid templates.
search.less adds styles for the filter sidebar, bucket counts, active

filter indicators, and the Contents filter checkbox group.
_comment.html and _group.html are new partial templates for

rendering comment and group search results inline.

Ran full test suite.

Verified that dispatch correctly routes to the faceted path when

Elasticsearch is available and falls back silently to the Haystack

path when it is not or when engine construction fails.
Verified template selection, user attachment on search hits, numeric

query redirection, and Whoosh wildcard rewriting.
Confirmed the existing Haystack path is unaffected.
Verified that review request, user, and group results all display

correctly in their respective datagrids with correct column data and

ES-ordered rows.
Verified that letter navigation on the users and groups tabs filters

correctly server-side via ES prefix/regexp queries and that the active

letter is correctly highlighted after navigation.
Verified that pagination reflects ES total counts rather than ORM

counts across all three result tabs.
Confirmed correct behaviour when switching between result tabs,

applying sidebar filters, and navigating pages.