Allow initial whitespace & omitted ending whitespace in levels

For example, this is Microban level 15 (indented by 4 spaces):

         ###
    ######@##
    #    .* #
    #   #   #
    #####$# #
        #   #
        #####

The previous regex removed initial whitespace, messing up the first and
last two rows. Also, we ended up with a non-rectangular board because
the implied space at the end of the first row is omitted from the level
.txt file. make-rectangular adds that.
Fix spurious <td> element

Oops. Remnant from early attempt using tables to render the board.
Check if data is loaded before rendering, or display loading msg
Async loading and add Microban level set
Add script for importing .txt files of Soko levels
Credit Microban set