Drop notebook numbering scheme.

There exists multiple branches,some with differing sets of notebooks. Maintaining a simple incrementing numbering scheme for notebooks results in the same notebooks having different titles across branches (and thus across H2O environments).

I propose to drop the number prefix altogether. Notebooks sourced from their respective repos likely don't carry the prefix - removing them would also mean a higher chance that titles are consistent across repos.

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sidenote: The colon character (`:`) is not a valid character for notebook _titles_ in some (most?) notebook engines. Replacing colons with something else (e.g., hyphens) in the notebook filenames would add consistency between a notebook's file name and it's title, as well as make the notebook compatible with more notebook engines.