make the codecogs equation base url a setting

fixes CNVS-10049

test-plan:

 * clear codecogs.equation_image_link setting
 * visit /equation_images/%5Csqrt%5B2%5D%7B900%7D%3Dx
 * should be redirected to http://latex.codecogs.com/gif.latex?%5Csqrt%5B2%5D%7B900%7D%3Dx
 * set codecogs.equation_image_link setting to "http://example.com/"
 * visit /equation_images/%5Csqrt%5B2%5D%7B900%7D%3Dx
 * should be redirected to http://example.com/?%5Csqrt%5B2%5D%7B900%7D%3Dx

Change-Id: Ic683111ec4109bacdd5c06c633c70129ab688fcc
Reviewed-on: https://gerrit.instructure.com/30212
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Cody Cutrer <cody@instructure.com>