Add core image server settings

[whikloj]

JIRA Ticket: https://jira.duraspace.org/browse/ISLANDORA-2469

Related PRs:

• Centralize Image server settings to Core. islandora_paged_content#158
• Move image server to core islandora_internet_archive_bookreader#136
• Move imageserver to core islandora_openseadragon#102

What does this Pull Request do?

Creates a new configuration space in core for image server settings. This will be shared between Paged Content, the IABV and Openseadragon.

What's new?

An admin form, tokens and some helper functions.

How should this be tested?

Try setting as type as none, Djatoka and IIIF, ensure the form saves your settings. Set using different tokens for IIIF, etc. If you have a custom setup for your site, try it here and ensure it is still supported.

Additional Notes:

Example:

• Does this change the interface, add a new feature, or otherwise change behaviours that would require updating documentation? Yes, documentation changes could be needed in the wiki. I have altered the old configuration forms to direct people here to configure their image server.
• Does this change add any new dependencies? no
• Does this change require any other modifications to be made to the repository (ie. Regeneration activity, etc.)? no
• Could this change impact execution of existing code? It will if settings are not auto-configured until someone fixes them.

Interested parties

@Islandora/7-x-1-x-committers @jonathangreen

[whikloj]

All good suggestions, thanks @adam-vessey. Updates coming.