refactor diff panel UI logic from file service into diff component

[WanniCode]

Resolves issue #230.

Note that:

• an inter component communication service was used to connect the file panel and diff panel components . a bit hacky perhaps, but was done so that minimal changes would be needed to the legacy file service API.

Testing:

1. Start the application
2. Continue without signing in (or sign in - this should not matter since we're just opening a local repository)
3. select a local repository
4. select any files that have changes from the file panel (on the left)
5. the diff panel should show green/red line changes as normal
6. editing should work

Note that step 3. can be replaced by any method of opening a repository (so it could also be cloned from a remote repository on GitHub first).

Essentially, for formality, regression tests would have to be done in order to verify the refactoring did not break the application in any way (relative to how it was before the refactor). At the code level (at the time of this PR), this means that non-breaking behavior should be observed from file.service.ts, file.panel.component.ts, diff.panel.component.ts, and diff-service.ts. The link between the file onClick() handler in file.component.ts and updating the diff panel component should still work.

[henryli333]

Can you also fix the missing function issue in header component? Relates to #242

[WanniCode]

made the changes suggested above. please check again :)

[WanniCode]

Can you also fix the missing function issue in header component? Relates to #242

this is gonna be fixed in a separate PR