Remove the "double space at end of line" markdown rule

[PhantomWatson]

The "put two spaces at the end of a line if you want to follow it with a single line break" markdown rule is not intuitive for most writers, and failure to understand this rule results in line breaks not appearing (which can really fuck up poetry).

If this rule were replaced with a simple nl2br() alternative, it wouldn't be a breaking change for existing thoughts.

• Update markdown-to-HTML conversion process
• Update markdown guide
• Clear all cached HTML content for thoughts, profiles, etc.

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Actually

A better solution to this would be to replace plain markdown editing with a markdown-compliant rich text editor. ToastUI Editor looks like a good option.