All <tables> go through MarkdownTable

What does this MR do and why?

Closes Pass all `<table>`s through `MarkdownTable` to ... (#630465).

All <tables> go through MarkdownTable.

render_markdown_tables.js now puts all tables through MarkdownTable except for those that legitimately don't want any treatment at all (already-applied, detached, JSON (to be fixed!), nested). We work out if the table is to be sorted here, but even non-sortable tables still get put into MarkdownTable.

We also fix how we determine the sortable header row: it can be in the <thead>, or it can be a single leading row of <th>s in the <tbody>, thus catching RTE-emitted HTML-syntax tables.

Screenshots or screen recordings

What	Before	After
1. Pipe table syntax in the PTE
		No change.
2. <table><thead><tr><th>...</th>...</tr></thead><tbody><tr>...</tr>... in the PTE (table has <thead> with one <tr> containing <th>s, and <tbody>)
		No change.
3. <table><tr>...</tr><tr>...</tr>... (table has no <thead>, no <tbody>, no <th>s at all) in the PTE
		Table gains wrapper (note top outline), but not sortable, as there is neither <thead> nor <th> in the source.
4. <table><tr><th>...</th>...</tr><tr>...</tr>... in the PTE (table has first <tr> containing only <th>s)
		Table is now sortable!
5. Regular table created in the RTE
		No change.
6. Table created in the RTE with block-level elements (when you receive the "HTML warning" while editing)
		Table is now sortable!
7. AsciiDoc wiki page with striped header table, and striped headerless table
		Striped headerless table gets wrapper.
8. <table><thead><tr>...</tr><tr>...</tr></thead><tbody>... (table has two <tr>s in <thead>)
		Table doesn't lose its second header row (!); no longer sortable.
9. Case 4 in a Rapid Diffs MR diff comment
		Table is now sortable!

How to set up and validate locally

TODO

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