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

[rwinch]

Thanks for the PR. I've left a few comments inline.

I also wonder if for the tests (that we have updated) if we can reduce the use of + and \n on the large Strings? I can understand the desire to avoid using the templates in the tests, but perhaps """ can still be used with minimal + of the variables. Alternatively, if we know the value of the variables we could just use a large constant there and assert that the value of the variables matches our constant.

[rwinch]

I'm curious if there is a need for the \s escape sequence in the Strings? Is this related to the checkstyle problems?

[Kehrlann]

Almost, the problem is the formatter, not checkstyle. Spring-javaformat deletes trailing spaces on lines, but not \s. Checkstyle does not complain.

Our text blocks do contain some blank lines with spaces, mostly due to calling .indent(...) on the CSS style blocks. So in our tests, we need to preserve the blank, non-empty lines in text-blocks. The vast majority will go away once we move the CSS out into a "static" resource.

[Kehrlann]

Addressed, using text blocks in tests too.