Fix: Inserting email template

[mheiduk]

Description

When selecting an email template on the 'send invoice' page, it won't get inserted into the textarea. val() instead of html() has to be used here.

Motivation and Context

Fixes the above mentioned issue.

Pull Request Checklist

• My code follows the code formatting guidelines.
• I have an issue ID for this pull request.
• I selected the corresponding branch.
• I have rebased my changes on top of the corresponding branch.

Issue Type (Please check one or more)

• Bugfix
• Improvement of an existing Feature
• New Feature

[nielsdrost7]

Thanks for the PR, it's merged into the development branch.

[bminer]

Does this solve #962?

[nielsdrost7]

@bminer This PR was created in December of 2022 and issue #962 was created after that.
For the issue with the <style> tags in the email templates there are 2 solutions:

1. Documentation: "Please do not use style tags in your templates"
2. strip the style tags after the user submits his template

We'll fist have to reproduce the problem thanks to your excellent steps to reproduce.
You can try the solution (see the files tab), but I think we need to wait for the solution where we strip the styles from the html

[bminer]

I helped create #962, and it applies to 1.6.0. This PR #900 was incorporated into one of the 1.6.0-beta versions that I've not tried. That's why I was asking.