CON-1658: remove outdated -webkit CSS browser prefixes

Remove ``-webkit-box-sizing`` and ``-webkit-box-shadow`` from our SASS mixins.

Pinging @kvas-damian  and @lizlux 

```
Offenders for cssOldPropertyPrefixes (6):
 * input[type="search"] { -webkit-box-sizing: content-box } // was required by android 3, chrome 9, ios_saf 4.3, safari 5 and earlier @ 1:1602
 * .WikiaArticle .ForumActivityModule { -webkit-box-shadow: 0px 2px 5px 0px #e5d5c6 } // was required by android 3, chrome 9, ios_saf 4.3, safari 5 and earlier @ 1:645
 * .UserLoginDropdown .WikiaForm .input-group { -webkit-box-sizing: border-box } // was required by android 3, chrome 9, ios_saf 4.3, safari 5 and earlier @ 1:1046
 * .UserLoginDropdown .WikiaForm input[type=text], .UserLoginDropdown .WikiaForm input[type=password], .UserLoginDropdown .WikiaForm input[type=submit] { -webkit-border-radius: 0 } // was required by android 2.1, chrome 4, ios_saf 3.2, safari 4 and earlier @ 1:1582
 * .UserLoginDropdown .WikiaForm input[type=text], .UserLoginDropdown .WikiaForm input[type=password] { -webkit-box-sizing: border-box } // was required by android 3, chrome 9, ios_saf 4.3, safari 5 and earlier @ 1:1731
 * .UserLoginDropdown .WikiaForm input[type=text], .UserLoginDropdown .WikiaForm input[type=password] { -webkit-box-shadow: none } // was required by android 3, chrome 9, ios_saf 4.3, safari 5 and earlier @ 1:1799
```