Resolve "Tweak migrated.html template for migration fail cases"

Closes #26 (closed) Edit: This MR covers a little more than was described in #26 (closed). Things that were done:

1. added white-space wrapping for html pre tags - used to display stdout, stderr and kea generated config
2. introduced jinja html template inheritance - less html redundancy and common styling for all pages displayed to the user
3. introduced flask url_for - better URLs maintenance
4. simplified share_file form
5. tweaked migrated.html template - display content only when available
6. obfuscate file paths displayed in production in stderr and stdout
7. py and html code cleaning, typos fixing etc.