Make CI and pre-push hooks more efficient.

Currently, Oppia has a pre-push hook which runs frontend unit tests on all files and lacks any backend unit tests in the pre-push hook which can lead to a lack of response for developers until it reaches the Github CI. Furthermore backend unit tests take quite long and similarly End-to-End, lighthouse and acceptance tests on the Github CI are all ran regardless of the PR's changes; all of these problems lead to developer distress. This proposal seeks to solve these problems by running frontend unit tests only on changed files and adding backend unit tests in the pre-push hook, to run similarly on only changed files. It also seeks to reduce Github CI and pre-push hook runtimes by optimizing backend unit tests. Alongside this, this proposal will reduce Github CI usage by running End-to-End, lighthouse and acceptance tests on only PRs which need them.