Explicit "text post" support recast as generic "post" support.

First of an expected two commits implementing the restructuring promised in the immediately previous Commit 3b611b02a3.

Highlights:
- `Blog#new_post` no longer uses `TextPostBuilder` to build a post, but instead calls #post_source (which is a wrapper around Post.new);
- The `Support::PostParamParser` class no longer supports a :post_type attribute;
- The `Post` class has a :body attribute again, which must be present for the instance to be valid (for now);
- Goodbye, module `PostTextSupport` and `Support::TextPostBuilder`.

RSpec: 113 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 42 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1