[Feature] Support spellchecking of pages

[stevejalim]

In a line: As a site maintainer, I want site-checker to verify spellings on all pages, so that I can see if there are any typos to correct.

Context

When we have large content drops (eg State of Mozilla) and as we move towards more CMS-held content on mozilla.org, we would benefit from a layer of verification of the site's content so that we can catch and fix typos promptly.

Notes

• We'll start with en-US only for now, because that's the source of truth for most (but not all) localised pages. We should build with an eye on multi-locale support

## Success criteria

• We have an appropriate base dictionary to use for en-US.
• There's a configurable user dictionary to which we can add custom terms
• Issues are automaticaly raised in github.com/mozilla/bedrock (ideally) detailing any misspelled words