Add a Text Format helper [#3332053]

Problem/Motivation

It would be great if there was support for making changes to the HTML tag restrictions of Text Formats more easily. Currently, allowed HTML tags and their attributes are a simple string of text for example <span class> <a href class> <img alt> <p>

This makes adding new tags, or adding/removing attributes hard to do programmatically.

Proposed resolution

Add a Text Format service that adds some helper functions to make this easier.

User interface changes

None

API changes

New service for adding/removing tags and attributes.

Data model changes

None

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3332053-add-a-text changes, plain diff MR !15
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