Use correct logic with custom URL placeholder

[RohanNagar]

This issue relates to a:

• Bug
• Suggestion
• Feature Proposal

Description:

Continuation of #150. The user should be able to (optionally) customize the string that is used as a placeholder for the URL. Here are the following scenarios and what should happen:

1. No urlPlaceholderString defined and no custom email pages defined

   Use the default CODEGEN-URL and the default HTML/Text files

2. Custom urlPlaceholderString (not equal to CODEGEN-URL) and no custom email pages

   Log warning and use CODEGEN-URL anyway with the default files

3. No urlPlaceholderString defined and one or both custom email pages defined

   Use default CODEGEN-URL and the custom pages or one custom page and one default

4. Custom urlPlaceholderString (not equal to CODEGEN-URL) and one custom email page

   Use the custom placeholder for the one page that is custom, use CODEGEN-URL for the other

5. Custom urlPlaceholderString (not equal to CODEGEN-URL) and both custom email pages

   Use the custom placeholder for both custom pages

Additional Information:

[RohanNagar]

Also want to customize email subject. Want to start using a EmailMessage class with a builder pattern. Also want a separate configuration object within ses for message options, and want configuration to look like:

ses:
  endpoint:
  region:
  fromAddress:
  messageOptions:
    subject:
    bodyHtmlFile:
    bodyTextFile:
    urlPlaceholderString:
    successHtmlFile:

[RohanNagar]

Work started on the email branch

More experimentation on this - taking notes here for me to remember

Create class MessageOptionsConfiguration - holds the config
Create class MessageOptions - holds the values and will be provided by EmailModule based on the config, taking care of defaults and all that
VerificationResource will take in MessageOptions and use that instead of taking in individual strings.

[RohanNagar]

Thought this was done in #172, but realized that I didn't take care of the special scenarios outlined in the main post. Keeping this open until those are done.

[RohanNagar]

Being worked on in the placeholder branch.