Startup time improvements

[rtibbles]

Summary

• Limit imports of pkg_resources to defer a costly import
• Move code out of module scope and into the core app ready method
• Defer actual registration of zeroconf to the RUN event so that it doesn't block SERVING
• Default the no_input kwarg of initialize to True - this will not effect cli invocations that provide their own default value, but will help apps that are calling the initialize function and which are running in no_input contexts.

In all, these changes seemed to give approximately a 60-70% reduction in time taken from first start to being ready to serve HTTP requests.

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Testing checklist

• [x] Contributor has fully tested the PR manually
• [ ] If there are any front-end changes, before/after screenshots are included
• [ ] Critical user journeys are covered by Gherkin stories
• [x] Critical and brittle code paths are covered by unit tests

PR process

• [x] PR has the correct target branch and milestone
• [x] PR has 'needs review' or 'work-in-progress' label
• [x] If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• [ ] If this is an important user-facing change, PR or related issue has a 'changelog' label
• [ ] If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions[bot]]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0.dev0_git.20220924000318.pex
Unsigned Windows installer	kolibri-0.16.0.dev0+git.20220924000318-unsigned.exe
Debian Package	kolibri_0.16.0.dev0+git.20220924000318-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0.dev0+git.20220924000318-0.3.0.dmg
Source Tarball	kolibri-0.16.0.dev0+git.20220924000318.tar.gz
WHL file	kolibri-0.16.0.dev0+git.20220924000318-py2.py3-none-any.whl

[rtibbles]

Irritatingly, a change in here seems to be breaking the Mac App build. Will investigate further before we merge this.

[rtibbles]

DMG build failure appears to be unrelated to these changes. Tracked separately: https://github.com/learningequality/kolibri-app/issues/105