Style redirect link differently

* Make the redirect target on mappings grey to aid scanning the
mappings list
* It also declutters the list and makes it clear which part is
responsible for the mapping
* Grey matches existing redirect text
Replace HTTP status codes with words

* 301 is now Redirect, 410 is Archive
* Move edit link to right hand column, aligned with add mapping button
and the check mapping option on hits
* Make edit link a button
* Rename HTTP status column to type
* Add the same redirect/archive colour coding as used on hits to
distinguish mapping types
Fix margin helper class

* Use the right margin property
Rename mappings title to include hostname

* Matches new sites table
Update mapping form to use words not codes

* Use the same terminology as the mappings page, but keep the HTTP
meanings for the more technical
* Use bootstrap form pattern to clarify path field
* Rename label to Old URL now the form is clearer
* Distinguish between saving and creating on the form button
* Include helpful placeholders
Explain HTTP code in abbreviation

* Mitigate some of the confusion about hit status codes by including
the default meaning as a title attribute
Correct vertical spacing of mapping form
Remove HTTP status from mappings form

* Update corresponding tests