Fix typo in .env_sample file for the AWS secret access key variable

References Coursemology/coursemology.org#287
Merge pull request #299 from fonglh/dotenv-typo

Fix typo in .env_sample file for the AWS secret access key variable
Fixed mobile chrome bug for rich text editor
Merge pull request #301 from llja0112/mobile-chrome-bug-fix

Fixed mobile chrome bug for rich text editor
limit python eval result length
limit python eval result length(2)
display stack trace when student attempt
display test result limit actually expression length