feat(Bandcamp): set creative commons license URL if available

[phw]

This extracts Creative Common license URLs from Bandcamp pages and submits them as license URL link to MB.

This also generally completes the support for external links of type "license". Fixes #19

Some examples for testing:

• https://aeonsable.bandcamp.com/album/aenigma-2023
• https://aneagleinyourmind.bandcamp.com/album/intersection
• https://deusexlumina.bandcamp.com/track/my-church-is-black-me-and-that-man
• https://binaerpilot.bandcamp.com/album/songs-for-alan-turing

[phw]

As a side note I think it might be helpful to actually handle the HTML response with a HTML parser and be able to use CSS path queries on it. Would lead to more robust code than the current regular expression based approach.