Use longer HTTP timeouts during migrations.

Since we're not so bothered about a quick response during migrations, and more
bothered about whether the migration completes successfully or not, it makes
sense to go for a higher timeout on these requests.
Merge pull request #156 from alphagov/migration-timeouts

Use longer HTTP timeouts during migrations.
Fix timeout test by stubbing, not mocking.

The mocked `index_name` call in the indexing test is called by a piece of
logging code, so may or may not be called depending on the environment. It's
not important to the function of the code itself, so we can stub it (no
expectation it will be called) rather than mocking it.