[ticket/11935] Fix <p> tags not allowed inside <strong> tags

PHPBB3-11935
[ticket/11935] Fix incorrect alt tags that should be title tags

PHPBB3-11935
[ticket/11935] Fix stray </a> tags

PHPBB3-11935
[ticket/11935] Fix invalid HTML usage of <label> tag in prosilver

PHPBB3-11935
[ticket/11935] Remove obsolete table attributes from Prosilver

cellpadding, cellspacing, border are obsolete table attributes.
They are handled by CSS already using table and .table1

PHPBB3-11935
[ticket/11935] Replace obsolete table attributes in Admin with CSS

PHPBB3-11935
[ticket/11935] Tweak colour swatch table

PHPBB3-11935
Merge branch 'develop' into ticket/11935
[ticket/11935] Merge remote-tracking branch 'upstream/develop'

PHPBB3-11935
Merge pull request #1788 from VSEphpbb/ticket/11935

[ticket/11935] Fix invalid HTML and Errors