Change Canonical URI statement to https://

[jcowey]

Wherever the Canonical URI statement appears in the form of e.g. https://papyri.info/ddbdp/bgu;2;363

Canonical URI: http://papyri.info/ddbdp/bgu;2;363

Please change the statement to

Canonical URI: https://papyri.info/ddbdp/bgu;2;363

At present http redirects through to https, but the statement of it being https would be nicer.

picks up on #266.

When this is done it should probably be made to coincide with a re-index. The last big http => https change caused quite a few problems at the time.