Preserve line breaks in markdown

[nknguyenhc]

Summary:

Fixes #1230

Changes Made:

• Allow preserving line breaks. This is done by changing the breaks option in the app module markdown declaration.

Note that in #1230, the subset list is not exactly rendered as a list, but is rendered with linebreaks.

Github preserves linebreaks when rendering markdown, hence it looks like the items are rendered as list items.

One consequence of this PR is that, whenever two sentences are separated only by one line break, in html, instead of the two being one paragraph (as expected from standard markdown syntax), the two are preserved and rendered as two paragraphs (as expected behaviour of github markdown).

Summarising everything in one example, the following markdown code:

1. Item 1
  A. Sub item 1
  B. Sub item 2

| Header 1 | Header 2 |
|---|---|
| Cell 1 | Content 2 |

* Name
* Space

Line 1.
Line 2.

Paragraph 1.

Paragraph 2.

is rendered as:

Proposed Commit Message:

Preserve line breaks

With preserving linebreaks, subset list items are rendered as a list,
and paragraph rendering is the same as Github.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2c18b71) 53.41% compared to head (3c4af54) 53.44%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1241 +/- ## ========================================== + Coverage 53.41% 53.44% +0.03% ========================================== Files 103 103 Lines 2928 2928 Branches 544 544 ========================================== + Hits 1564 1565 +1 Misses 1014 1014 + Partials 350 349 -1 ```

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[nknguyenhc]

Also, I think you can explore upgrading our ngx-markdown version to enable gfmd as you mentioned in #1230

Actually I realised that ngx-markdown documentation was actually outdated/incorrect. Also, GFMD was already enabled in CATcher, so right now the current ngx-markdown version is sufficient. We probably can explore upgrading ngx-markdown version when we upgrade the packages/angular/node version.