Use HTML templating in default UIs

[Kehrlann]

reviewer: @rwinch

Some notes:

• ~I have not updated the tests, they still use string concatenation sometimes. I'd rather not use the templating in the tests themselves.~ Using text blocks and constants in tests.
• I have added tests that render the full document and assert equality. While they are brittle, they allow to iterate much faster by seeing what's broken.
• ~For context, checktyle / SpringLeadingWhitespace breaks when using space-indented text-blocks. There is an open issue in spring-javaformat ; //@formatter:off does not suppress the error. I have re-indented all text blocks with tabs. If we really prefer spaces, then we would need file-wide checkstyle suppressions.~ This is is Fixed in spring-javaformat 0.0.43

[Kehrlann]

Addressed, using text blocks in tests too.