Add disclaimer to default welcome text

[bagofarms]

Add a disclaimer to the bottom of the default welcome text that reads:

Please Note: This tool is meant to be used as a guide, not a certification. It only checks for common accessibility issues, and is not comprehensive; a clean report in UDOIT does not necessarily mean that your course is fully accessible. Likewise, the tool may indicate a possible accessibility issue where one does not exist.

[emilysachs]

Resolved in this commit 4815495#diff-0c2c7c4d879752ee30b98fc11328c9fd