Fix browsers blocking GET requests on files/html due CORS

[fabiodrg]

• Fixes the development environment issue where unit tests would not work because of CORS when fetching HTML files dynamically. Closes #93
• Adjust timeouts on some unit tests
• Fixes table validation in Datatable extractor ( a bit off-topic, but...)
• Attempts to extract the last row if its a footer. For now, only supports rows with an explicit Sigarra's class that indicates the row shows a total value. By removing the rows from the body, such rows are not considered for sorting/filtering, which I think is a nice touch
• Ensures the DOM remains untouched if the DataTable fails by cloning the original <table> node and making all pre-processing and DataTable init in the cloned version. Only when there is a guarantee the table is valid, the DOM is updated.
• Fixes loading GIF not showing in infinite scroll after first page loading
• Enhances how new pages data are inserted in the table in infinite scroll: rather than destroying the table, it uses the existing APIs to add the new fetched items
• Disables data table and infinite scroll in pages where we know in advance they will fail and could conflict with other extractors: bills, time table and exams

[fabiodrg]

I am converting this to a draft because I detected a potential conflict with datatables and other extractors. For instance, the exam extractor. I will look into it later today. I will probably push into this PR because I did some fixes in datatables here, soo.. just preventing git conflicts

[fabiodrg]

Issue went away. Indeed applying DataTable was clashing with the bills extractor. But since DataTable fails, after performing the cleaning, the bills extractor still works. Nonetheless, I think we should review all extractors contexts and block DataTables if (a) data tables don't make sense on some specific page (i.e. exams page) or if there is any chance of conflicts (e.g. bills, which in fact we know in advance it will fail anyway). Will do that tomorrow