Allow customization of "Read more" link

Problem/Motivation

Currently all slides have a "Read more" link. While this works, in some cases it would be preferable to have a customized like text like "Read the report" or "Join" or "Count me in".

Also, the current implementation is not readily translatable.

Proposed resolution

We're using a link field here, so to make messages customizable we would:

• Enable link title. Possible issue: long titles could break display, e.g., increase chance of description text overlapping button. Therefore, set a low max length of 35 characters.
• Set default to "Read more" via a form alter hook implementation. This is necessary because of #1321094: Allow setting a default Title without setting a default URL.
• Edit existing view display to use title rather than hard-coded text "Read more".

Remaining tasks

User interface changes

API changes

Data model changes