Add html files to the file types that Google can convert. This allows
the Google Drive converter to convert online submissions to pdf. This
fixes an issue where if online submissions is selected and the Google
Drive converter is used all conversions fail and produce a single
blank pdf. Credit to Peter Svec for the idea for this fix.
Significant string changes:
* errorinvalidbyhour,core_calendar - correcting number from 59 to 23
* cron_help,core_admin - updated recommendation to run cron every minute
* tour1_content_end, tour1_content_welcome, tour2_content_end,
tour2_content_welcome all in tool_usertours - removing reference to
3.2
* invalidsesskey,core_error and invalidsesskey,mod_wiki - more
understandable error message
* pluginname_help and pluginnamesummary in qtype_essay - explaining that
the question type can be used for file uploads
* formatnoinline, mustattach and mustrequire in qtype_essay - changing
the word 'inline' to 'online'
