Revert "Revert "Merge pull request #107 from alphagov/spelling_suggestions""

Reintroduce the spelling suggestions.

This reverts commit b7cff2514f43101ef370369cf8795051e43f1908.
Specify an explicit encoding for Aspell.

Without this, Aspell tries to fall back on determining a locale from the
environment: if this fails (as it can do when running within Upstart), it
returns a value of "none" for the encoding, which blows up when ffi-aspell
tries to use it.

This is unfortunately tricky to test without delving deep into the Aspell
library, so a strongly-worded comment and the knowledge that Preview will show
up any failures will have to do for now.
Fix comment typo.
Clarify which requests go through the Suggester.
Merge pull request #113 from alphagov/fix-suggestion-encoding

Re-merge suggestions with encoding fix