Clean up "String continuations" warnings

[sritchie]

The following are emitted when evaluating mathbox code in cljs:

------ WARNING #35 - ---------------------------------------------------------- Resource: node_modules/shadergraph/src/visualize/markup.js:131:22 String continuations are not recommended. See https://google.github.io/styleguide/jsguide.html#features-strings-no-line-continuations

------ WARNING #36 - ---------------------------------------------------------- Resource: node_modules/shadergraph/src/visualize/markup.js:132:56 String continuations are not recommended. See https://google.github.io/styleguide/jsguide.html#features-strings-no-line-continuations

------ WARNING #37 - ---------------------------------------------------------- Resource: node_modules/shadergraph/src/visualize/markup.js:140:22 String continuations are not recommended. See https://google.github.io/styleguide/jsguide.html#features-strings-no-line-continuations

------ WARNING #38 - ---------------------------------------------------------- Resource: node_modules/shadergraph/src/visualize/markup.js:143:56 String continuations are not recommended. See https://google.github.io/styleguide/jsguide.html#features-strings-no-line-continuations