Add non-www support to all the pre-emptive rules as per #113

- Only duplicate one test case per non-www rule.
For completeness, add non-www support to the fallback rules too

- Only duplicate one test case per non-www rule.
Reformat fallback rules

* From `if`/`elsif` to nested `case`
* Consistent with preemptive rules
Merge pull request #116 from alphagov/fallback-rules-readability

Reformat fallback rules
Merge pull request #114 from alphagov/add_non-www_support_to_preemptive_rules

Add non-www support to all the pre-emptive and fallback rules
Limit sitemaps to 50,000 entries

Bouncer's sitemaps include all redirect mappings from that site. This causes
three problems for sites with large numbers of redirect mappings:
* the request takes too long to process, so a 504 is served
* our memory usage jumps on each occasion for the largest sites
* we are violating the sitemap specification, which limits each file to 50,000
  entries

The only downside to this change is that some redirects will not be included in
sitemaps, but we think this is better than not being able to serve a sitemap at
all, or Bouncer running out of memory.

If you need more than 50,000 entries, you should use multiple sitemap files,
like GOV.UK: https://www.gov.uk/sitemap.xml
Merge pull request #117 from alphagov/limit_sitemap_to_50_000

Limit sitemaps to 50,000 entries