v3.1.1 with plenty of bug fixes

[Fixed] version control of update urls
[Fixed] cache control of json profile
[Fixed] view of skinlib search
[Fixed] invisible language select on mobile
[Fixed] compatibility of String.prototype.endsWith
[Fixed] bug when getting type of real-time preview
[Fixed] url() function when using CDN
[Fixed] minus page numbers
[Fixed] checking in after zero
[Fixed] getting environment variables
[Fixed] translation of noticing wrong password
[Fixed] showing flash message
[Fixed] i18n when http exception occurs
[Fixed] getting database prefix
[Improved] invalid length of default APP_KEY
[Improved] update phpdotenv to v2.4.0
[Improved] translations of error pages
[Improved] regenerate session id after logging out
[Improved] add notice for adblock