Remove Edition.with_content_containing

Commit a90087a5ae1fc0e6dda89ad3b25e70db654d9b16 replaced the only use of this
method with Edition.with_summary_containing.
Rename Edition scope to make it clearer what it does

The scope in question searches *both* title and summary attributes and this
wasn't clear from the original name - `Edition.with_summary_containing`.

Commit a90087a5ae1fc0e6dda89ad3b25e70db654d9b16 makes it clear that this
behaviour is intentional.
Hardcode 'editions' table name

It's not clear why we're using `table_name` in these queries. As far as we can
tell, `table_name` will always be 'editions'. Hardcoding it makes it more
explicit.

We found the first one of these scopes added in
64eee76c54ef6c3af533a133a79842cd851304d8 but it didn't contain any information
as to why `table_name` was used.