archivebot.py: count size by bytes instead of the number of characters

Using UTF-8 encoded bytes conforms with units used on MediaWiki history
pages and with the bot's documentations in the script and on-wiki.

Change-Id: I0f1170de4fb9856afbb63aa8e654dac5c9de0884
Merge "archivebot.py: count size by bytes instead of the number of characters"