Add upload of mail templates

[GoebelL]

Summary Allow service manager to customize mail templates for the approval tool. Therefore enhance the approval controller to provide for uploading of mail templates that can be used in the technical service.

Details Provide File Upload option through the oscm-app-approval UI.

• Use AppTemplateService to read and write the template files from and to the APP database.
• Enhance ApprovalInstanceAccess#BasicSettings with a method getMailTemplate to obtain the content of configured mail template
• Adapt EmailWriter to to use the mail body from above getMailTemplate method.

Acceptance criteria

• [ ] Service manager of the approval tool can provide own HTML mail templates.
• [ ] Mail template is configurable via parameters in the technical service template
• [ ] If no mail template is configured, default mail template is used
• [ ] If configured template is not found or failed to load, default mail template is used
• [ ] If configured template is not found or failed to load, a clear message is found in the logs

Additional context Ask me

[GoebelL]

Implemented with Customizable approval requests #55 https://github.com/servicecatalog/oscm-app/pull/111