Support for customised URLs at platform level

Notes
In some cases the URL provided for a resource on a platform needs to be customised to be the correct URL to use for the resource within a particular institution. These customisations happen for all URLs on a single platform. An example is:

Platform: Ciando
Original URL: http://ebooks.ciando.com/book/index.cfm?bok_id=2795522
Customised URL used by State and University Library Bremen: http://suub-bremen.ciando.com/book/index.cfm?bok_id=2795522
Customised URL used by State and University Library Hamburg: http://sub-hh.ciando.com/book/?bok_id=2795522

The two customised URL examples come from what would be separate tenants.
Further examples of customised URLs (at platform level) are available in the spreadsheet at https://docs.google.com/spreadsheets/d/1Yqy5gOd_6O_Ys_EpdPs5nUsBUVO1OCCPmslWQhT62f0/edit?usp=sharing

In order to support this we will need:

• UI to find, view and edit Platforms
• A new field at platform level to store a "local platform code" which institutions can use to store a local code which they use to identify the platform (this is for use in either customised URLs or proxied URLs. Examples are given in spreadsheet linked above)
• The ability to create one-to-many Platform URL customisation settings per platform (decision on how this works pending ERM-1125)

New user permissions will be required. From the UI perspective these will be:

• Agreements: Search and view platforms
• Agreements: Edit platforms

Deliverables / Outputs
New Search and sort for Platforms (within the Agreements UI) (Done)
New view screen for Platforms (Done)
New edit screen for Platforms (Done)
New "Platform local code" field for Platform (Done)

To be done as part of separate story:

• Ability to add one or more URL customisation settings to a platform