Use `node.text` instead of `node.to_html` in mention-filters

[ashmaroli]

Because

• the filters are only doing simple text substitution on the inner-text.
• calls :to_html involves allocating and empty options hash on each node.
• already in use by similar filter (emoji_filter): https://github.com/jch/html-pipeline/blob/8f9e95fa7b1af0410092d8f0a908c977b00989ac/lib/html/pipeline/emoji_filter.rb#L18-L28