2 line breaks = new paragraph

In a previous feature (#2761) I requested removing the auto-double space feature when selecting the Enter key. This was particularly problematic for mobile devices, since a single line break with Shift + Enter was not possible. Ultimately, it was decided not to change this behavior for web, but for mobile it was modified such that two spaces would equate to a new paragraph (#2775).

I'm wondering if we can at least make a modification in web where two single spaces (Shift + Enter twice) will convert to a new paragraph. Or put differently, Shift + Enter twice is treated the same as Enter once.

There are three main reasons for this.

1. It flows with the traditional markdown logic, whereby a line break between two lines of texts signifies a new paragraph.
2. This logic will be consistent with the new behavior in mobile (#2775).
3. It creates less mental burden on the user. There are times when I would like to create two spaces in between paragraphs, to signify a new section/chapter. Currently, to do this properly I will have to hit Shift + Enter once, and then Enter once -- two separate actions. Otherwise, it won't be treated as a new paragraph and formatting will carry over. This is an extra mental step that seems unnecessary.

Note: for users that like the auto-double spacing with the Enter key, this won't change anything.

No file or test is named in the issue. Start by locating the web editor's handling of Enter and Shift+Enter, then compare it with the mobile behavior described in #2775. Done means two Shift+Enter actions create the same paragraph boundary as one Enter without changing existing single-Enter behavior.