Improve displaying of 'quoted' messages.

[sumanthvrao]

This PR focuses on tracking improvement to issue of displaying 'quoted text' in a better manner.

• The first approach here was to indent quoted text based on the level of quoting. Multi line texts are padded with a padding character.
• V2: Use LineBox approach to display the quoted messages; We could split up the html for different block-quotes and apply soup2mark on each of these.

[sumanthvrao]

@zulipbot add "next major release"

[neiljp]

@sumanthvrao You're working on this now, right?

[sumanthvrao]

@neiljp Yes, I am working on this now. Could you please assign it to me?

[sumanthvrao]

The first of these - 'Indenting' quoted text based on level of quoting is now merged as e7421af and e422fb8 to reduce spacing at the start of the block-quote messages.