feature: markdown field (easy_mde replacement)

[adrianthedev]

Description

Fixes #3378

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - https://docs.avohq.io/3.0/fields/markdown.html
• I have added tests that prove my fix is effective or that my feature works
• fix i18n tasks
• check if redcarpet is installed when trying to use the field

Screenshots & recording

Manual review steps

1. Go to a project
2. test the field in description

• write
• preview
• styles
• paste a file with cmd+V
• drop a file over the field

Manual reviewer: please leave a comment with output from the test if that's the case.

[codeclimate]

Code Climate has analyzed commit 05e719e and detected 0 issues on this pull request.

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[adrianthedev]

Let's wait a bit with this one. I have an idea.

[adrianthedev]

Closing in favor of https://github.com/avo-hq/marksmith