Use "c-format" flag instead of "javascript-format" in POT

[lslezak]

Problem

• Originally we used a simple javascript format flag which basically supported only simple %s or %d formats
• But since we switched to the sprintf-js library (see Use the sprintf-js library instead of format-util #735), which supports full C-like format strings, we can use the c-format flag in the POT file
• So do not change the language flag and keep the original c-format used by xgettext
• Additionally the translators might be more familiar with the C format than the Javascript format

Testing

• Tested manually, the generated POT file contains the c-format flags
• I manually updated the existing PO files and all translations still validate (via msgfmt --check-format)

[coveralls]

coverage: 72.814%. remained the same when pulling 5e4ea91 on pot_langugage_attribute into 5612a99 on master.