Only open and write to webpage once its contents have been filled

When generating a summary webpage, currently all the individual pulsar/detector page html files get opened together. This can lead to a lot of open files, which can easily exceed the normal open file limit (see ulimit -n). However, it is not necessary for all the files to be open and continually be written to with new content. Instead the file path, rather than an open file pointer, and content for each page can be stored, and the file opened for writing just once. This MR does this.