Remove dependencies on Linguist

This saves about 128 MB of baseline RAM usage per Unicorn and
Sidekiq process (!).

Linguist wasn't detecting languages anymore from CE/EE since
9ae8b574. However, Linguist::BlobHelper
was still being depended on by BlobLike and others.

This removes the Linguist gem, given it isn't required anymore.
EscapeUtils were pulled in as dependency, but given Banzai depends on
it, it is now added explicitly.

Previously, Linguist was used to detect the best ACE mode. Instead,
we rely on ACE to guess the best mode based on the file extension.