Restructure modals and base template

app/main/routes.py

app/templates/_base/_modals/_manual/_01_introduction.html.j2

..._base/_modals/_manual/_02_registration_and_log_in.html.j2

app/templates/_base/_modals/_manual/_03_dashboard.html.j2

app/templates/_base/_modals/_manual/_06_services.html.j2

.../_manual/_07_a_closer_look_at_the_corpus_analysis.html.j2

...ates/_base/_modals/_manual/_08_cqp_query_language.html.j2

...templates/_base/_modals/_manual/_09_query_builder.html.j2

app/templates/_base/_modals/_manual/_10_tagsets.html.j2

app/templates/_base/_modals/manual.html.j2

app/templates/_base/_modals/terms_of_use.html.j2

app/templates/_base/footer.html.j2

app/templates/_base/modals.html.j2

app/templates/_base/navbar.html.j2

app/templates/_base/roadmap.html.j2

app/templates/_base/scripts.html.j2

app/templates/_base/sidenav.html.j2

app/templates/_base/styles.html.j2

app/templates/base.html.j2

app/templates/main/user_manual.html.j2