investigate adding config for suppressing tech notes entirely

[davewhiteland]

Currently, tech notes are either published on the server (automatically — they're part of the server repo), or are forwarded to an external URL (which is indicated by TECH_NOTES_EXTERNAL_URL not being empty). (I think that external URL does preserve the page path when forwarding.)

To suppress tech notes entirely means also removing the link to tech notes on the project page: would need to have something like IS_TECH_NOTES_DISPLAYED in order to suppress it entirely. (If this existed, could also warn (perhaps?) if any tech note URLs are in the task list HTML.