Field settings CAN be changed

Problem/Motivation

It's possible to change the 'allowed number of values' from Unlimited to Limited on different types of fields applied to different types of entities, so it seems that this is a generic problem. The warning that the field settings can no longer be changed is displayed, and other configuration elements applicable to that field are disabled.

Tested at the moment:
- User
- taxonomy term
- plain text
- Node
- taxonomy term

Data loss is not immediate, and as long as the data in the field in question is not modified in any way, it's possible to change it back to unlimited and recover the original data. If the field data is modified, any data not shown in the UI will be deleted.

Proposed resolution

Still investigating

Remaining tasks

1. Write patch
2. Review

User interface changes

None, except that the 'allowed number of values should be shown either as a disabled select box or static content.

API changes

None

Data model changes

None