#1277 Change how images are credited

[Ahasuerus]

Change how images are credited on Publication pages. As per a May 2019 Community Portal discussion, the new behavior should be as follows:

1. Clicking a cover scan (including SFE3-hosted cover scans) should always take the user to the raw image file.

2. Modify the credit line to have two links instead of one. The first link will continue to be a link to the site's home page. The second, new, link will be to a publication-specific page (if defined.)

3. The new wording of the credit line will be "Cover art supplied by X on this Web page" where:

   "X" will be the name of the organization hyperlinked to its home page, and
   "this Web page" will be a link to a publication-specific page, specifically:
   for SFE3-hosted images it will be the cover scan's Gallery page
   for ISFDB-hosted images it will be the ISFDB Wiki page of the cover scan
   for Science Fiction-Leihbuch-Datenbank-hosted images it will be the publication-specific page associated with the cover scan
   all other cover scans won't have the words "on this Web page" displayed because there is no page to link to

For now we will keep the current way of entering links to publication-specific pages, which uses "|" to separate the cover scan URL from the URL of the publication-specific page. If the current proposal is accepted and implemented, I expect that we will revisit this issue at some point in the future and probably create a separate field for the data that is currently entered to the right of "|", but that's another discussion.

[Ahasuerus]

Implemented in:

biblio/biblio.py
biblio/common.py
biblio/pl.py

Installed in SVN 409 on 2019-05-08. Closing the FR.