Merge branch 'bugfix/html-only-mail' into 'master'

add parsing support for incoming html email

## What does this MR do?
Fixes #18388 by adding support for parsing HTML email

## Are there points in the code the reviewer needs to double check?
The new class, Gitlab::Email::HTMLParser, which needs to translate the HTML content to text and also delete replies, as they are not necessarily in the correct format to be caught by EmailReplyParser. The solution I found that should work for any HTML-formatted email is to remove all `<table>` and `<blockquote>` tags. Actual `<table>` elements (to be interpreted by markdown) should already be encoded with e.g. `&lt;table&gt;` - the only failure mode is if there is an *actual* HTML table in the content itself, which we wouldn't be able to support easily anyways.

The gem `html2text` traverses the HTML tree and outputs text - and markdown in the case of HTML links or images.

See merge request !7397

1 files changed, 1 insertions, 0 deletions

diff --git a/Gemfile b/Gemfile
index 327d35c2350..9e815925a1f 100644
--- a/Gemfile
+++ b/Gemfile
@@ -330,6 +330,7 @@ gem 'octokit', '~> 4.3.0'
 gem 'mail_room', '~> 0.9.0'
 
 gem 'email_reply_parser', '~> 0.5.8'
+gem 'html2text'
 
 gem 'ruby-prof', '~> 0.16.2'