Initial work on adding an email sender field.

[kusold]

This separates the username from the sending address. This allows mail services such as SendGrid to be used.

Related Issue: #88

Some issues with this PR:

• It is not backwards compatible. Current installations won't have the sender field set. How have you done migrations in the past? Alternatively, I can do something like: sender: $advanced['mail']['sender'] || $advanced['mail']['username']
• This field currently shows for all providers. Can you point me to how you hide smtp server and port for predefined providers?
• This adds a new field to the gui. What is your preferred text? How do localizations work?

Let me know if you have any other feedback. Thanks for this project. :-)

[ninthwalker]

did some work in the v2-dev branch if you wanted to build and try. works for the most part, need to tweek a couple things. took some of your suggestions and built on it to make work. I can give more details if youd like to understand the process more. Thanks for the pull requests.

[kusold]

I just tested out your branch and the email went through.

I did find two issues though:

• Sendgrid was not an option on the initial setup screen.
• It looks like for sendgrid you hardcoded username to be apikey. Since you did that, you can hide the username field when sendgrid is the provider.