Improve report filter formatting

Problem/Motivation

The report's filter area is large and can be a bit difficult to read since the labels and form elements all run together. Also, there probably should be no 'Paragraph Type' filter if paragraphs is not installed or not in use in any layout.

Proposed resolution

1. Use 'container' instead of fieldset to be more consistent with other core reports
2. Position labels above form elements (form--inline)
3. Move buttons to an actions container
4. Don't show 'Paragraph Type' filter unless paragraphs is in use -- e.g. if $paragraph_types_set is empty.

Remaining tasks

• Open MR
• Test
• Merge

User interface changes

Improved filter area formatting.

API changes

None

Data model changes

None