This commit adds support for bylines to the Text Introduction block
in the same way that Item Introduction blocks support them. To avoid
duplication, the common byline functionality has been pulled out into
a new byline template (although not into a new Wagtail block).

All three of the affected templates (byline, item introduction, text
introduction) can be previewed using this repository's template debug
functionality at these URLs:

- http://localhost:8000/admin/template_debug/v1/byline/
- http://localhost:8000/admin/template_debug/v1/item_introduction/
- http://localhost:8000/admin/template_debug/v1/text_introduction/

Like Item Introduction bylines, Text Introduction bylines can have
zero or more authors and an optional date.

This commit also includes some minor cleanup around the way that
item introduction blocks have been manually rendered on various page
types. As far as I can tell this special handling is no longer needed,
if it ever was. Affected page types and example pages to verify this:

- BlogPage: http://localhost:8000/about-us/blog/why-were-modernizing-how-we-collect-credit-card-data/
- DocumentDetailPage: http://localhost:8000/compliance/supervisory-guidance/bulletin-phone-pay-fees/
- EnforcementActionPage: http://localhost:8000/enforcement/actions/capital-one-bank/
- LearnPage: http://localhost:8000/enforcement/information-industry-whistleblowers/privacy-act-statement/
- NewsroomPage: http://localhost:8000/about-us/newsroom/cfpb-issues-guidance-to-address-shoddy-investigation-practices-by-consumer-reporting-companies/
- HMDAHistoricDataPage: http://localhost:8000/data-research/hmda/historic-data/

This commit also adds a few new Python unit tests to validate the
intended template rendering.