Fix TinyMCE initialization for admin edit pages loaded with Turbolinks

TinyMCE settings are loaded in javascript_pre_init, however this was previously in <head>. Because Turbolinks is used, scripts in the <head> are only executed on the first page load or during a page refresh. Therefore, scripts that change on each page, need to be moved into <body>. To prevent executing shared libraries on each page load, data-turbolinks-eval=false is added to the main alchemy/alchemy javascript_include_tag. Alchemy.Tinymce must also not be overwritten if it's already assigned because ALchemy.Tinymce.init is defined in alchemy.tinymce.js.coffee which is only executed on the first page load.