Implement remaining features present in Java implementation

[jia1]

• [x] Add upload feature (fixes #18)
• [x] Auto-populate authentication and endpoint fields with a secret file (fixes #17)
• [x] Add default values for file paths (fixes #22)
• [x] Add link to online version of the email (fixes #24)
• [x] Add README for installation and usage
• [x] Add visual alerts for success/failure when sending email (fixes #19)
• [x] Add visual alerts for success/failure when uploading files (fixes #23)
• [x] Add image upload online preview
• [x] Fix logo being too close to the header (fixes #27)
• [x] Auto-upload generated HTML to server (fixes #28)

[jia1]

Please merge to master only when #29 is merged into this branch.