Various UX improvements

Problem/Motivation

I presented this module's UI to many of my colleagues and gathered some feedback to improve the user experience.

Steps to reproduce

Proposed resolution

• Change the action label from "Update terms" to "Update taxonomy terms"
• Change the confirmation page title from "Which term reference fields do you want to update?" to "Which taxonomy term assignments do you want to update?"
• On the form, for each taxonomy term field presented, indicate what content types(s) the field applies to. This should especially be helpful if you have selected many nodes of different content types.
• On the form, remove the sentence "Select actions and values for each field and these will be applied only to the content items that have that field.". Our users found this redundant and/or confusing.
• On the form, change the option "Replace with selected term(s)" to "Replace all with selected term(s)"
• On the form, change the option "Clear existing value" to "Clear all existing term(s)"
• On the form, for each taxonomy field presented, add an option "No changes" and have it selected by default.
• On the form, change the autocomplete field to a multi-select field. (This one we should make configurable. We would further enhance this with a JS library to improve the multi-select, but I imagine most sites won't and the standard multi-select is not very usable) (Do this in a separate issue: #3536405: Support multi-value select for term assignment as an alternative to the autocomplete input)

Remaining tasks

User interface changes

Various minor changes. Here's a screenshot that shows the changes described above:

API changes

Data model changes