Mock out environment in fact check email test.

The test would previously break if run with the wrong value of the
GOVUK_APP_DOMAIN environment variable.
Remove the TagRepository
Merge pull request #42 from alphagov/fact_check_email_environment

Mock out environment in fact check email test.
Remove unused and `Tag.id_and_entity` method.
Use methods on Tag to load tags and tag lists.

Mostly this just factors out the common occurrences of `Tag.where(...).first`
in the codebase, but it also lets us load a list of tags with a single Mongo
query using the `Tag.by_tag_ids` method or its corresponding bang method.
Use `for_tag_ids` in Taggable for loading.
Port existing methods and tests from Publisher
Fix bug where the body of licences weren't indexed
Remove nonsensical indexable_content logic

AFAICT this was excluding the body if there were any newer editions
(published or unpublished). Couldn't find anything explaintative in
the Git log for this repo of Publisher.
Fix nonsensical (out of date?) code

It's the job of the code doing the registration to determine if an
edition should be indexed, so it doesn't make sense for the edition
to behave differently if it's not published.
indexable_content includes body, if there is one
Remove markup from licence_overview for search
Remove markup from transaction text for search
Merge pull request #44 from alphagov/cleanup_and_fix_indexable_content

Cleanup and fix indexable content
Release 2.7.0
Add an index for tag_id for performance boost

We used to have a (unique) index on tag_id, which was swapped in
favour of an index on tag_id, tag_type.

Unfortunately, this massively slowed registering of (publisher) content
and probably other things as well.
Release 0.7.1
Merge pull request #43 from alphagov/remove_tag_repository_rebased

Replace the tag repository
Release 3.0.0.

Since we're removing the TagRepository module, which some dependent
projects are using in their tests, we should bump the major version.
Merge remote branch 'origin/master' into add-travel-advice-edition