Proper fix for changeRange() shrinking lines / Ctrl-L to clear REPL Window / Fix issue 303

[cfehse]

What has Changed?

• Fixed the error in webview/model.ts changeRange() properly when shrinking the lines due to a change. This was the root cause for the occasionally failing history commands.
• Added the Ctrl-L command to clear the REPL window from within the window to the shortcuts in the HTML intro.
• Made changes to the HTML intro to be a bit cleaner and made the logos a bit less prominent. All logos now have a hover to show the meaning of the logo (the calva logo links to the user documentation). I think this may be a good compromise to fix issue REPL colours and logo get too much attention #303.

Fixes #303

My Calva PR Checklist

I have:

• Read How to Contribute.
• Made sure I am directing this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I am changed the default PR base branch, so that it is not master. (Sorry for the nagging.)
• Tested the VSIX built from the PR (well, if this is a PR that changes the source code.) You'll find the artifacts by clicking Show all checks in the CI section of the PR page, and then Details on the ci/circleci: build test. (For now you'll need to opt in to the CircleCI New Experience UI to see the Artifacts tab, because bug.)
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.

The Calva Team PR Checklist:

Before merging we (at least one of us) have:

• Made sure the PR is directed at the dev branch (unless reasons).
• Read the source changes.
• Given feedback and guidance on source changes, if needed. (Please consider noting extra nice stuff as well.)
• Tested the VSIX built from the PR (well, if this is a PR that changes the source code.)
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
• If need be, had a chat within the team about particular changes.

Ping @PEZ, @kstehn, @cfehse