fix: namespace css classes

In order to reduce CSS clashes when injecting onto other pages, we are adding extra scope to make the selectors more unique.

Closes #32 (closed)

What's in this MR?

• Most CSS classes are now prefixed by #gitlab-review-container.

// Before
.gitlab-button-secondary:hover {
  // [...]
}

// After
#gitlab-review-container .gitlab-button-secondary:hover {
  // [...]
}

• The classes that are applied directly onto the container have an extra selection:

// Before
.gl-fixed {
  // [...]
}

// After
#gitlab-review-container .gl-fixed,
#gitlab-review-container.gl-fixed {
  position: fixed;
}

• README instructions in how to test the package locally with a GitLab instance (there might be better ways to go about this!)