Refactor the anchorencode function

Closes #31

See merge request !29

Fixes #31

Fixes #34

Add secure headers

Closes #28

See merge request !28

Only allow content from ourself and disable iframes, javascript and <base> elements.

Closes: #28

Add Referrer-Policy, X-Content-Type-Options and X-XSS-Protection.

By loading the XframeOptionsMiddleware django sets X-Frame-Options to
DENY preventing the website to be framed.

Updates for v1.1

Closes #25 and #24

See merge request !27

see #24

This redirects users directly to the man page if there is a match for
the query, otherwise it opens the search form as usual.

fixes #25

Deployment

See merge request !26

Fix typo in footer

Both title as well as id can still contain characters like '&', but
these strings can be considered safe since they were actually extracted
from HTML.